Vaccines have been used for many years to protect humans and animals against a wide variety of infectious diseases. Such conventional vaccines consist of attenuated pathogens (for example, polio virus), killed pathogens (for example, Bordetella pertussis) or immunogenic components of the pathogen (for example, diphtheria toxoid and hepatitis B surface antigen).
Some antigens are highly immunogenic and are capable alone of eliciting protective immune responses. Other antigens, however, fail to induce a protective immune response or induce only a weak immune response. The immune response of a weakly immunogenic antigen can be significantly enhanced if the antigens are co-administered with adjuvants. Adjuvants enhance the immunogenicity of an antigen but are not necessarily immunogenic themselves. Adjuvants may act by retaining the antigen locally near the site of administration to produce a depot effect facilitating a slow, sustained release of antigen to cells of the immune system. Adjuvants can also attract cells of the immune system to an antigen depot and stimulate such cells to elicit immune responses. Adjuvants have been identified that enhance the immune response to antigens delivered parenterally.
Adjuvants are commonly employed with antigen in vaccine formulations whereby the induction of systemic immunity through parenteral immunization (intramuscular or subcutaneous) is obtained. This approach is suitable for infectious agents gaining access to the body via damaged skin (i.e. Tetanus), however, there are problems encountered due to side-effects and associated toxicity of many adjuvants administered in this fashion. Only those vaccines formulated from aluminum salts (aluminum phosphate or aluminum hydroxide) find routine use in human and veterinary vaccination. However, even these adjuvants are not suitable for use with all antigens and can also cause irritation at the site of injection. There is a clear need to develop adjuvants which safely enhance the immunogenicity of antigens at the site of injection.
There are other problems specific to the nature of the antigen being used. For example most conventional non-living vaccines require multiple doses for effective immunization. Live attenuated vaccines and many nonliving liquid vaccines suffer from the need for controlled storage conditions and are susceptible to inactivation (e.g. thermal sensitivity). There are also problems associated with combining vaccines in single dosage forms, due to adjuvant incompatibilities, pH, buffer type and the presence of salts.
Mucosal immunity is induced primarily by induction of secretory immunoglobulin (sIgA) in intestinal, bronchial or nasal washings and other external secretions. For example parenteral cholera vaccines have been shown to offer limited protection whereas the more recently developed oral form is highly effective (ref. 1--throughout this specification, various references are referred to in parenthesis to more fully describe the state of the art to which this invention pertains. Full bibliographic information for each citation is found at the end of the specification, immediately preceding the claims. The disclosures of these references are hereby incorporated by reference into the present disclosure). Studies with human volunteers have shown that oral administration of influenza vaccine is effective at inducing secretory anti-influenza antibodies in nasal secretions and substances have been identified which might be useful as adjuvants for such ingested vaccines. However, most of these adjuvants are relatively poor in terms of improving immune responses to ingested antigens. Currently, most of these adjuvants have been determined to be safe and efficacious in enhancing immune responses in humans and animals to antigens that are administered via the orogastrointestinal, nasopharyngeal-respiratory and genital tracts or in the ocular orbits. However, administration of antigens via these routes is generally ineffective in eliciting an immune response. Although the above example illustrates the potential of these immunization modes, the development of vaccine formulations for use by these routes has been slow for various reasons. The inability to immunize at the mucosal surface is generally believed to be due to include:
(i) antigen degradation via the acid and/or proteolytic enzymes present during the transit to the mucosal surfaces; PA0 (ii) antigen degradation by secretions presented at the mucosal epithelium; PA0 (iii) limited adsorption across the mucosal epithelium; PA0 (iv) the dilution of the antigen to a concentration that is below that required to induce immune responses; and PA0 (v) ineffective adjuvants and/or delivery systems.
The problems associated with the use of adjuvants in parenteral vaccine formulations and the lack of suitable systems for vaccine delivery to mucosal sites understates the need for new techniques that are effective when administered by various routes and are inherently free from associated toxicity concerns or side-effects.
It is also desired to provide vaccine delivery in a single dosage form for both human and animal immunizations as this has the advantage of reducing time and cost, and in human medicine, increases patient compliance which is of extreme importance in developing countries where access is restricted. This is especially true for infants within these countries.
In order to increase immune responses to administered antigens, a carrier may be used to protect the antigen from degradation and also modulate the uptake of these materials in vivo. Sensitive antigens may be entrapped to protect them against destruction, reduction in immunogenicity or dilution. Methods for formulating a carrier include dispersing an antigen within a polymeric matrix (monolithic matrix) or by the coating of a polymeric material around an antigen to give an outer protective wall (core-shell). The manipulation of the formulation protocol can allow for control over the average size of these materials. This has been shown to be important for the uptake of particulates via oral delivery at specialized M-cells of the Peyers patches within the intestinal tract.
U.S. Pat. No. 5,151,264 describes a particulate carrier of a phospholipid/glycolipid/polysaccharide nature that has been termed Bio Vecteurs Supra Moleculairs (BVSM). The particulate carriers are intended to transport a variety of molecules having biological activity in one of the layers thereof. However, U.S. Pat. No. 5,151,264 does not describe particulate carriers containing antigens for immunization and particularly does not describe particulate carriers for immunization via the orogastrointestinal, nasapharyngeal-respiratory and urogenital tracts and in the ocular orbits or other mucosal sites.
Eldridge et al. (refs 2 and 3) observed the delayed release of antigen in vivo from biodegradable microspheres manufactured from polylactide-co-glycolide copolymer also known as PLG or PLGA. Numerous other polymers have been used to encapsulate antigens for formulation into microparticles and some of these include polyglycolide, polylactide, polycaprolactone, polyanhydrides, polyorthoesters and poly(8-hydroxybutyric acid).
U.S. Pat. No. 5,075,109 describes encapsulation of the antigens trinitrophenylated keyhole limpet hemocyanin and staphylococcal enterotoxin B in 50:50 poly (DL-lactide-co-glycolide). Other polymers for encapsulation are suggested, such as poly(glycolide), poly(DL-lactide-co-glycolide), copolyoxalates, polycaprolactone, poly(lactide-co-caprolactone), poly(esteramides), polyorthoesters and poly(8-hydroxybutyric acid), and poly anhydrides. The encapsulated antigen was administered to mice via gastric intubation and resulted in the appearance of significant antigen-specific IgA antibodies in saliva and gut secretions and in sera. As is stated in this patent, in contrast, the oral administration of the same amount of unencapsulated antigen was ineffective at inducing specific antibodies of any isotype in any of the fluids tested. Poly(DL-lactide-co-glycolide) microcapsules were also used to administer antigen by parenteral injection.
Published PCT application WO 91/06282 describes a delivery vehicle comprising a plurality of bioadhesive microspheres and antigenic vaccine ingredients. The microspheres being of starch, gelatin, dextran, collagen or albumin. This delivery vehicle is particularly intended for the uptake of vaccine across the nasal mucosa. The delivery vehicle may additionally contain an absorption enhancer. The antigens are typically encapsulated within protective polymeric materials.
U.S. Pat. No. 5,571,531 describes particulate carriers comprising a solid matrix of a polysaccharide and a proteinaceous material. A functionalized silicone polymer is bonded to the matrix for the delivery of materials having biological activity.
Although time-delayed release of antigen was shown in the above work, difficulties were encountered when microparticles are manufactured by the described methods. The exposure of biological materials to the organic solvents and physical forces used can lead to denaturation. It may be also be difficult to scale-up the procedures. Furthermore, hydrophilic antigens may be inefficiently encapsulated.
It would be desirable to provide improved carriers without such limitations. It would be particularly desirable to provide polymeric materials which can be formulated into microparticles and microspheres and which contain targetting moieties to target the antigen to preselected ligands. This would have tremendous potential for cells of the immune system.